Mining Deepslate takes longer than expected when the player is under the effect of haste.

According to all sources, Deepslate should take about 2 times longer to mine than stone. However, when under the effect of a Haste II beacon, the time to mine deepslate takes many times (5-8 times depending on technique) longer than mining stone.

I believe the effect is due to the game adding additional mining time to blocks which are not broken in 1game tick (instant mining). When stone is mined while the player is under haste II effect, the time to break stone is 1 game tick. However, Deepslate when mined under haste II has a 2 game tick delay, plus an additional 6 game ticks added on because it isn't being broken instantly.

I tested mining a 10x10x4 cube of material while under haste II effect and the results were that I mined out 400 stone in 30 seconds but the Deepslate took 166 seconds. I used the same pickaxe and the same technique (as best I could).

This is 5.5 times longer than it took to mine the stone. Since it has been stated that Deepslate should take only twice as long, I believe this to be a bug.

Important note: This effect is not seen when not under Haste Effect. The effect is best seen when mining a lot of blocks in a compact space. Mining the blocks while sprint moving forward seems to lessen the effect as well.